- Email: [email protected]
W free layer stacks
Thin Solid Films 587 (2015) 39–42
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Ta thickness-dependent perpendicular magnetic anisotropy features in Ta/CoFeB/MgO/W free layer stacks SeungMo Yang a, JaBin Lee a, GwangGuk An a, JaeHong Kim b, WooSeong Chung c, JinPyo Hong a,b,1 a b c
Novel Functional Materials and Devices Lab, The Research Institute for Natural Science, Department of Physics, Hanyang University, Seoul 133-791, South Korea Division of Nano-Scale Semiconductor Engineering, Hanyang University, Seoul 133-791, South Korea Nano Quantum Electronics Lab, Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, South Korea
a r t i c l e
i n f o
Available online 29 November 2014 Keywords: Perpendicular magnetic anisotropy Annealing stability Ta thickness
a b s t r a c t We describe Ta underlayer thickness influence on thermal stability of perpendicular magnetic anisotropy in Ta/CoFeB/MgO/W stacks. It is believed that thermal stability based on Ta underlay is associated with thermally-activated Ta atom diffusion during annealing. The difference in Ta thickness-dependent diffusion behaviors was confirmed with X-ray photoelectron spectroscopy analysis. Along with a feasible Ta thickness model, our observations suggest that an appropriate seed layer choice is needed for high temperature annealing stability, a critical issue in the memory industry. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Spin-Transfer-Torque Magnetoresistive Random Access Memory (STT-MRAM) has garnered considerable attention as a promising next generation nonvolatile memory due to extremely low power consumption, and high-speed performance [1,2]. In particular, STT-MRAM, including out-of-plane magnetized tunnel junctions (p-MTJ) is highly promising, because it suggests that it is possible to achieve the thermal stability factor Δ, which determines memory retention and write current density (Jc) [3]. Given these perpendicular magnetic anisotropy (PMA) material advantages, various PMA materials such as rare-earthtransition metal films, [Co/(Pt, Pd)] and [Fe/(Pt, Pd)] multilayers, or their L01 ordered alloy films, and CoFeB/MgO stacks have been widely explored for the past decade, since these materials show large PMA characteristics [4–6]. Among these PMA materials, CoFeB/MgO free layer stacks are considered the most viable candidate for STT-MRAM devices, due to their large tunneling magnetoresistance ratio, and low Gibert damping constant (α) [7]. Interface PMA properties in CoFeB/MgO stacks have often been explained by introducing the concept of Fe/Co 3d and O 2p orbital hybridization at the interface between the CoFeB layer and MgO barrier [8,9]. However, a critical issue for CoFeB/MgO stacks, including a Ta seed layer, is PMA feature degradation during annealing temperatures greater than 300 °C, which is equivalent to the annealing temperature used in a standard Complementary Metal–Oxide– Semiconductor integrated process [10]. Recently, N. Miyakawa et al. [11] reported that PMA feature deterioration in Ta/CoFeB/MgO stacks was a consequence of Ta invasion into the CoFeB/MgO interface, thus hindering Fe/Co and O atom hybridization. Therefore, Ta thickness
1
E-mail address: [email protected] (J. Hong). Tel.: +82 2 2220 0911; fax: +82 2 2296 3738.
http://dx.doi.org/10.1016/j.tsf.2014.11.068 0040-6090/© 2014 Elsevier B.V. All rights reserved.
dependence in Ta/CoFeB/MgO stacks, in terms of annealing-driven Ta diffusion behaviors above 300 °C, must be tested for optimum seed layer choice or development. In this paper, we describe Ta seed layer thickness influence on magnetic characteristics of the Ta/CoFeB/MgO configuration at annealing temperatures up to 375 °C, where Ta thickness was varied from 0.75 nm to 4.5 nm. Sample magnetic features, including different Ta thicknesses, were measured utilizing a vibrating sample magnetometer (VSM). In addition, atomic depth profile analyses for Ta and Co atoms were carried out employing an X-ray photoelectron spectroscopy (XPS) system for making annealing-driven Ta atom diffusion event observations using a Theta Probe XPS instrument from Thermo Fisher Scientific. 2. Experimental details Ta/Co20Fe60B20/MgO/W stacks were prepared on thermally oxidized Si substrates utilizing a radio-frequency (RF) magnetron sputteringsystem at an Ar 4 × 10−1 Pa working pressure, with a base pressure of less than ~2.5 × 10−5 Pa. Deposition rates for each layer were carefully determined using an Atomic Force Microscopy system. Typical film surface roughness for each layer was less than 2.0 Å. The species was [Si/ SiO2] substrate/Ta (tTa)/Co20Fe60B20 (tCFB)/MgO (2)/W (3), where numbers in parenthesis refer to layer thickness in nanometers. Ta layer thickness (denoted as tTa) varied from 0.75 nm to 4.5 nm, with a 0.75 nm step, and CoFeB layer thickness was also changed from 1.02 nm to 2.22 nm. A W capping layer was used to prevent oxidation induced by atmosphere during measurement. To show Ta layer annealing influence on magnetic features, a post-annealing process was carried out at 300, 325, 350, and 375 °C for 1 h under vacuum conditions below ~ 1.3 × 10− 4 Pa, with a 3 tesla perpendicular magnetic field.
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Finally, in-plane and out-of-plane magnetic hysteresis loops were measured by VSM. 3. Results and discussion Fig. 1 plots the representative magnetic hysteresis loops of Ta (1.2 nm)/CoFeB (1.2 nm)/MgO (2 nm)/W (3 nm) stacks after thermal treatments under a 3 tesla perpendicular magnetic field at various temperatures (up to 375 °C). In all graphs, black and red lines represent in-plane and out-of-plane direction hysteresis loops, respectively. As expected [6], annealing at a moderate temperature (300 and 325 °C) revealed well-known PMA features in Ta/CoFeB/MgO stacks, as shown in Fig. 1a and b. However, as annealing temperature increased, PMA properties began to degrade, or their magnetic characteristics disappeared. This PMA feature deterioration after high temperature thermal treatment might originate from thermally activated Ta atom diffusion toward the CoFeB and MgO layers [11]. Interfacial anisotropy between oxide materials and ferromagnetic metals (CoFeB/MgO) is attributed to Co and Fe 3d orbitals and O 2p orbital hybridization. Therefore, thermally activated Ta atom diffusion into the interface between CoFeB and MgO may easily diminish PMA characteristics. Moreover, diffusion of a large number of Ta atoms into CoFeB can create an intermixed CoFeB–Ta layer, resulting in the absence of magnetic property [12]. Thus, the practical question about how Ta atom diffusion affects degradation of magnetic properties remains. Magnetization saturation (Ms) and effective anisotropy value (Keff) dependence on Ta thickness when CoFeB layer thickness was held constant (1.2 nm) is summarized in Fig. 2. Keff was determined from the area enclosed between the in-plane and out-of-plane hysteresis loops. As seen in Fig. 2a, Ms values decreased as Ta thickness increased when annealing temperature was held constant. In addition, as the annealing temperature increased, Ms decrease was larger in thicker Ta. These two observations suggest that thicker Ta layers diffused more toward the CoFeB and MgO layers. Similarly, Keff showed a similar tendency with respect to Ms changes: that is, a thicker Ta layer was associated with a reduced interfacial anisotropy due to disturbed bondings between Co and Fe 3d orbitals, and O 2p orbitals induced by Ta atom diffusion into
the CoFeB–MgO interface. This produced a lower Keff value. In this sense, an interesting result was observed, as seen in Fig. 2. A thick Ta layer easily induced fast PMA property degradation, or magnetic feature collapse at relatively lower annealing temperatures, suggesting a thickness-dependent diffusion process during annealing. A relatively thick Ta layer easily drove more diffusion. This phenomenon may be described as follows: for a diffusion event, Ta atoms have their own distribution probability functions determined by the Einstein diffusion relation, vacancy diffusion, or an exchange diffusion process [13]. In addition, Ta atom average amount at a location different from their initial location after annealing is the product of each distribution function, and initial Ta atom amount, which is dependent on thickness. Thus, a thick Ta layer easily provided a relatively large Ta atom amount at examined locations under the same annealing condition for 1 h, compared to a thin Ta layer. This is the reason a thick Ta layer revealed relatively unstable annealing features. In order to clarify Ta thickness-dependent diffusion behavior during thermal treatment, Co and Ta atom XPS depth profile analysis was performed in a low 10−6 Pa vacuum chamber. XPS spectra were taken in a constant analyzer energy mode with 100 eV pass energy using Al Kα radiation (15 kV, 6.7 mA). The etching process was carefully conducted with a time step of 3 s up to 51 s for Samples A and B using Ar ion etching (2 kV, 15 mA). All the XPS spectra were calibrated using C 1s (284.5 eV) [14] binding energy as a reference. Co 2p1/2 and Co 2p3/2 (793.6 eV and 778.5 eV) [15] peaks and Ta 4f5/2 and Ta 4f7/2 (23.4 eV and 21.9 eV) [16] peaks were analyzed using Avantage 5.91 software. Two Ta geometries are shown in Fig. 3: Ta (1.5)/CoFeB (1.2)/MgO (2)/W (3), and Ta (4.5)/CoFeB (1.2)/MgO (2)/W (3) stacks, denoted Samples A and B, respectively. Fig. 3b reveals typical depth profile analyses for Ta and Co atoms in Samples A and B annealed at 350 °C. Blue and green lines indicate Co atoms in Samples A and B, respectively, and black and red lines represent Ta atom distribution in Samples A and B, respectively. Co depth analyses (blue and green lines) for the two samples are almost identical after thermal treatment, clearly demonstrating that Co atom diffusion processes are almost the same, regardless of Ta thickness. In addition, the similar Co atom distributions means that etching level 5 for both samples represents the CoFeB/MgO interface region, as presented
Fig. 1. Magnetic hysteresis loops (black and red lines represent in-plane direction and out-of-plane direction, respectively) for various Ta thickness samples at different annealing temperatures (a) 300 °C, (b) 325 °C, (c) 350 °C and (d) 375 °C. Numbers in each graph indicate Ta layer thickness used in this experiment.
S. Yang et al. / Thin Solid Films 587 (2015) 39–42
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Fig. 3. (a) Ta (tTa)/CoFeB/MgO/W stack schematic illustration for XPS analysis, where tTa is 1.5 nm and 4.5 nm. (b) Depth profiling analyses for Ta and Co atoms in Samples A and B. Expected locations in real configurations along each etching time are shown behind the graph.
Fig. 2. (a) Magnetic moment per unit volume and (b) effective anisotropy energy density as a function of Ta layer thickness for Ta/CoFeB/MgO/W stacks at different annealing temperatures.
in Fig. 3b. However, a Ta 4f depth profile measurement revealed a clear difference in Ta atom distributions after thermal treatment. In particular, at the CoFeB/MgO interface region (12 s etching), a larger Ta atom amount in Sample B appeared, compared to those in Sample A, possibly confirming changes in effective anisotropy for both samples after thermal treatment (Fig. 2b). Therefore, distinctness in Ta depth profiles was possible indirect evidence for Ta thickness-dependent diffusion phenomena during annealing. A closer investigation for ultrathin Ta thickness samples (Ta 0.75 nm stacks) exhibited an easy transition from PMA to in-plane magnetic anisotropy features at the relatively lower annealing temperature of 325 °C, as shown in Fig. 4. This is different from the tendency shown in Fig. 2. The difference may be related to the fact that ultra thin Ta thickness (0.75 nm) provided an unclear Ta interface. It possibly led to a ragged CoFeB interface, inducing a lower interfacial anisotropy (Ki) value between CoFeB and MgO layers at a relatively lower annealing temperature. In addition, Ta atom diffusion influence in an ultra thin 0.75 nm sample is less than those in other thicker samples. However, since this sample's initial interface anisotropy feature was not better than thick samples, the 0.75 nm Ta thick sample did not allow for better PMA properties, even at the relatively lower 325 °C annealing temperature, compared to thick samples.
To confirm our observations for various Ta thicknesses, the deadlayer in the CoFeB layer was obtained as a function of CoFeB thickness for 0.75 nm, 1.5 nm, and 4.5 nm Ta thick samples, which were not thermally treated. As shown in Fig. 5, dotted lines represent first order linear fitting for each sample, and vertical solid lines, including corresponding numbers, indicate the representative dead-layer thicknesses at the linear fitting x axis-intercept. The dead layer thickness of the Ta 0.75 nm sample was 0.39 nm, whereas 1.5 nm and 4.5 nm samples revealed relatively thin 0.26 nm dead-layers. The thick dead-layer presence for the 0.75 nm sample could result from a large surface roughness, which might cause relatively low interface anisotropy value (Ki) due to rough interface between CoFeB and MgO layers.
4. Conclusion In conclusion, we report Ta thickness influence on Ta/CoFeB/MgO stack magnetic features at various annealing temperatures. Magnetic hysteresis loop observations and XPS atomic depth analysis suggested that the difference in Ta atom diffusion process depended on Ta layer thickness. In addition, ultra thin Ta layers were not useful to provide stable PMA characteristics after high temperature annealing, since thin seed layers easily induced a large surface roughness, such as islands. Therefore, a proper Ta thickness or other seed layer is required for high thermal stability realization.
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Fig. 4. Typical M–H Ta (0.75)/CoFeB (1.2)/MgO (2)/W (3) stack curve at various annealing temperatures (a) 300 °C, (b) 325 °C, (c) 350 °C, and (d) 375 °C.
Fig. 5. Magnetization saturation as a function of the CoFeB thickness for Ta (tTa)/CoFeB (x)/MgO (2)/W (3) frame, where tTa varied: 0.75 nm, 1.5 nm and 4.5 nm. Linear fit to the data is shown by dotted lines. Numbers with vertical lines indicate dead-layer thickness for different stacks.
Acknowledgments This work was partially supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2060350), and partially by the Global Ph.D. Fellowship Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2012002282). References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molna, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, X. Jiang, L. Gao, J.Z. Sun, S.P. Parkin, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488.
[2] Takayuki Kawahara, Riichiro Takemura, Katsuya Miura, Jun Hayakawa, Shoji Ikeda, Young Min Lee, Ryutaro Sasaki, Yasushi Goto, Kenchi Ito, Toshiyasu Meguro, Fumihiro Matsukura, Hiromasa Takahashi, Hideyuki Matsuoka, Hideo Ohno, 2 Mb SPRAM (Spin-Transfer Torque RAM) with bit-by-bit bi-directional current write and parallelizing-direction current read, IEEE J. Solid State Circ. 45 (2010) 869. [3] S. Mangin, D. Ravelosona, J.A. Katine, M.J. Carey, B.D. Terris, Eric E. Fullerton, Currentinduced magnetization reversal in nanopillars with perpendicular anisotropy, Nat. Mater. 5 (2006) 210. [4] K. Mibu, T. Shinjo, Magnetic anisotropy in Fe/rare-earth multilayers, Hyperfine Interact. 113 (1998) 287. [5] P.F. Carcia, Perpendicular magnetic anisotropy in Pd/Co and Pt/Co thin‐film layered structures, J. Appl. Phys. 63 (1988) 5066. [6] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H.D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, H. Ohno, A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction, Nat. Mater. 9 (2010) 721. [7] Shinji Yuasa, Taro Nagahama, Akio Fukushima, Yoshishige Suzuki, Koji Ando, Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions, Nat. Mater. 3 (2004) 868. [8] H.X. Yang, M. Chshiev, B. Dieny, J.H. Lee, A. Manchon, K.H. Shin, First-principles investigation of the very large perpendicular magnetic anisotropy at Fe/MgO and Co/MgO interfaces, Phys. Rev. B 84 (2011) 054401. [9] B. Rodmacq, S. Auffret, B. Dieny, S. Monso, P. Boyer, Crossovers from in-plane to perpendicular anisotropy in magnetic tunnel junctions as a function of the barrier degree of oxidation, J. Appl. Phys. 93 (2003) 7513. [10] Y. Fukumoto, H. Numata, K. Suemitsu, K. Nagahara, N. Ohshima, M. Amano, Y. Asao, H. Hada, H. Yodai, S. Tahara, Switching-field stabilization against effects of hightemperature annealing in magnetic tunnel junctions using thermally reliable NixFe100-x/Al-Oxide/Ta free layer, Jpn. J. Appl. Phys. 45 (2006) 3829. [11] N. Miyakawa, D.C. Worledge, K. Kita, Impact of Ta diffusion on the perpendicular magnetic anisotropy of Ta/CoFeB/MgO, IEEE Magn. Lett. 4 (2013) 1000104. [12] Soo Young Jang, Chun-Yeol You, S.H. Lim, S.R. Lee, Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure, J. Appl. Phys. 109 (2011) 013901. [13] Jorg Karger, Douglas M. Ruthven, Doros N. Theodorou, Diffusion in Nanoporous Materials, vol. 1Wiley-Vch, Weinheim, 2012. 25 (100–101). [14] G.K. Wertheim, P.M.Th.M. Van Attekum, S. Basu, Electronic structure of lithium graphite, Solid State Commun. 33 (1980) 1127. [15] A.B. Mandale, S. Badrinarayanan, S.K. Date, A.P.B. Sinha, Photoelectron-spectroscopic study of nickel, manganese and cobalt selenides, J. Electron Spectrosc. Relat. Phenom. 33 (1984) 61. [16] L.A. DeLouise, Investigation of the chemical mechanisms of Ta(110)/Ta(110)suboxide etch selectivity using C12 molecular beams, Surf. Sci. 324 (1995) 233.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Ta thickness-dependent perpendicular magnetic anisotropy features in Ta/CoFeB/MgO/W free layer stacks SeungMo Yang a, JaBin Lee a, GwangGuk An a, JaeHong Kim b, WooSeong Chung c, JinPyo Hong a,b,1 a b c
Novel Functional Materials and Devices Lab, The Research Institute for Natural Science, Department of Physics, Hanyang University, Seoul 133-791, South Korea Division of Nano-Scale Semiconductor Engineering, Hanyang University, Seoul 133-791, South Korea Nano Quantum Electronics Lab, Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, South Korea
a r t i c l e
i n f o
Available online 29 November 2014 Keywords: Perpendicular magnetic anisotropy Annealing stability Ta thickness
a b s t r a c t We describe Ta underlayer thickness influence on thermal stability of perpendicular magnetic anisotropy in Ta/CoFeB/MgO/W stacks. It is believed that thermal stability based on Ta underlay is associated with thermally-activated Ta atom diffusion during annealing. The difference in Ta thickness-dependent diffusion behaviors was confirmed with X-ray photoelectron spectroscopy analysis. Along with a feasible Ta thickness model, our observations suggest that an appropriate seed layer choice is needed for high temperature annealing stability, a critical issue in the memory industry. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Spin-Transfer-Torque Magnetoresistive Random Access Memory (STT-MRAM) has garnered considerable attention as a promising next generation nonvolatile memory due to extremely low power consumption, and high-speed performance [1,2]. In particular, STT-MRAM, including out-of-plane magnetized tunnel junctions (p-MTJ) is highly promising, because it suggests that it is possible to achieve the thermal stability factor Δ, which determines memory retention and write current density (Jc) [3]. Given these perpendicular magnetic anisotropy (PMA) material advantages, various PMA materials such as rare-earthtransition metal films, [Co/(Pt, Pd)] and [Fe/(Pt, Pd)] multilayers, or their L01 ordered alloy films, and CoFeB/MgO stacks have been widely explored for the past decade, since these materials show large PMA characteristics [4–6]. Among these PMA materials, CoFeB/MgO free layer stacks are considered the most viable candidate for STT-MRAM devices, due to their large tunneling magnetoresistance ratio, and low Gibert damping constant (α) [7]. Interface PMA properties in CoFeB/MgO stacks have often been explained by introducing the concept of Fe/Co 3d and O 2p orbital hybridization at the interface between the CoFeB layer and MgO barrier [8,9]. However, a critical issue for CoFeB/MgO stacks, including a Ta seed layer, is PMA feature degradation during annealing temperatures greater than 300 °C, which is equivalent to the annealing temperature used in a standard Complementary Metal–Oxide– Semiconductor integrated process [10]. Recently, N. Miyakawa et al. [11] reported that PMA feature deterioration in Ta/CoFeB/MgO stacks was a consequence of Ta invasion into the CoFeB/MgO interface, thus hindering Fe/Co and O atom hybridization. Therefore, Ta thickness
1
E-mail address: [email protected] (J. Hong). Tel.: +82 2 2220 0911; fax: +82 2 2296 3738.
http://dx.doi.org/10.1016/j.tsf.2014.11.068 0040-6090/© 2014 Elsevier B.V. All rights reserved.
dependence in Ta/CoFeB/MgO stacks, in terms of annealing-driven Ta diffusion behaviors above 300 °C, must be tested for optimum seed layer choice or development. In this paper, we describe Ta seed layer thickness influence on magnetic characteristics of the Ta/CoFeB/MgO configuration at annealing temperatures up to 375 °C, where Ta thickness was varied from 0.75 nm to 4.5 nm. Sample magnetic features, including different Ta thicknesses, were measured utilizing a vibrating sample magnetometer (VSM). In addition, atomic depth profile analyses for Ta and Co atoms were carried out employing an X-ray photoelectron spectroscopy (XPS) system for making annealing-driven Ta atom diffusion event observations using a Theta Probe XPS instrument from Thermo Fisher Scientific. 2. Experimental details Ta/Co20Fe60B20/MgO/W stacks were prepared on thermally oxidized Si substrates utilizing a radio-frequency (RF) magnetron sputteringsystem at an Ar 4 × 10−1 Pa working pressure, with a base pressure of less than ~2.5 × 10−5 Pa. Deposition rates for each layer were carefully determined using an Atomic Force Microscopy system. Typical film surface roughness for each layer was less than 2.0 Å. The species was [Si/ SiO2] substrate/Ta (tTa)/Co20Fe60B20 (tCFB)/MgO (2)/W (3), where numbers in parenthesis refer to layer thickness in nanometers. Ta layer thickness (denoted as tTa) varied from 0.75 nm to 4.5 nm, with a 0.75 nm step, and CoFeB layer thickness was also changed from 1.02 nm to 2.22 nm. A W capping layer was used to prevent oxidation induced by atmosphere during measurement. To show Ta layer annealing influence on magnetic features, a post-annealing process was carried out at 300, 325, 350, and 375 °C for 1 h under vacuum conditions below ~ 1.3 × 10− 4 Pa, with a 3 tesla perpendicular magnetic field.
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Finally, in-plane and out-of-plane magnetic hysteresis loops were measured by VSM. 3. Results and discussion Fig. 1 plots the representative magnetic hysteresis loops of Ta (1.2 nm)/CoFeB (1.2 nm)/MgO (2 nm)/W (3 nm) stacks after thermal treatments under a 3 tesla perpendicular magnetic field at various temperatures (up to 375 °C). In all graphs, black and red lines represent in-plane and out-of-plane direction hysteresis loops, respectively. As expected [6], annealing at a moderate temperature (300 and 325 °C) revealed well-known PMA features in Ta/CoFeB/MgO stacks, as shown in Fig. 1a and b. However, as annealing temperature increased, PMA properties began to degrade, or their magnetic characteristics disappeared. This PMA feature deterioration after high temperature thermal treatment might originate from thermally activated Ta atom diffusion toward the CoFeB and MgO layers [11]. Interfacial anisotropy between oxide materials and ferromagnetic metals (CoFeB/MgO) is attributed to Co and Fe 3d orbitals and O 2p orbital hybridization. Therefore, thermally activated Ta atom diffusion into the interface between CoFeB and MgO may easily diminish PMA characteristics. Moreover, diffusion of a large number of Ta atoms into CoFeB can create an intermixed CoFeB–Ta layer, resulting in the absence of magnetic property [12]. Thus, the practical question about how Ta atom diffusion affects degradation of magnetic properties remains. Magnetization saturation (Ms) and effective anisotropy value (Keff) dependence on Ta thickness when CoFeB layer thickness was held constant (1.2 nm) is summarized in Fig. 2. Keff was determined from the area enclosed between the in-plane and out-of-plane hysteresis loops. As seen in Fig. 2a, Ms values decreased as Ta thickness increased when annealing temperature was held constant. In addition, as the annealing temperature increased, Ms decrease was larger in thicker Ta. These two observations suggest that thicker Ta layers diffused more toward the CoFeB and MgO layers. Similarly, Keff showed a similar tendency with respect to Ms changes: that is, a thicker Ta layer was associated with a reduced interfacial anisotropy due to disturbed bondings between Co and Fe 3d orbitals, and O 2p orbitals induced by Ta atom diffusion into
the CoFeB–MgO interface. This produced a lower Keff value. In this sense, an interesting result was observed, as seen in Fig. 2. A thick Ta layer easily induced fast PMA property degradation, or magnetic feature collapse at relatively lower annealing temperatures, suggesting a thickness-dependent diffusion process during annealing. A relatively thick Ta layer easily drove more diffusion. This phenomenon may be described as follows: for a diffusion event, Ta atoms have their own distribution probability functions determined by the Einstein diffusion relation, vacancy diffusion, or an exchange diffusion process [13]. In addition, Ta atom average amount at a location different from their initial location after annealing is the product of each distribution function, and initial Ta atom amount, which is dependent on thickness. Thus, a thick Ta layer easily provided a relatively large Ta atom amount at examined locations under the same annealing condition for 1 h, compared to a thin Ta layer. This is the reason a thick Ta layer revealed relatively unstable annealing features. In order to clarify Ta thickness-dependent diffusion behavior during thermal treatment, Co and Ta atom XPS depth profile analysis was performed in a low 10−6 Pa vacuum chamber. XPS spectra were taken in a constant analyzer energy mode with 100 eV pass energy using Al Kα radiation (15 kV, 6.7 mA). The etching process was carefully conducted with a time step of 3 s up to 51 s for Samples A and B using Ar ion etching (2 kV, 15 mA). All the XPS spectra were calibrated using C 1s (284.5 eV) [14] binding energy as a reference. Co 2p1/2 and Co 2p3/2 (793.6 eV and 778.5 eV) [15] peaks and Ta 4f5/2 and Ta 4f7/2 (23.4 eV and 21.9 eV) [16] peaks were analyzed using Avantage 5.91 software. Two Ta geometries are shown in Fig. 3: Ta (1.5)/CoFeB (1.2)/MgO (2)/W (3), and Ta (4.5)/CoFeB (1.2)/MgO (2)/W (3) stacks, denoted Samples A and B, respectively. Fig. 3b reveals typical depth profile analyses for Ta and Co atoms in Samples A and B annealed at 350 °C. Blue and green lines indicate Co atoms in Samples A and B, respectively, and black and red lines represent Ta atom distribution in Samples A and B, respectively. Co depth analyses (blue and green lines) for the two samples are almost identical after thermal treatment, clearly demonstrating that Co atom diffusion processes are almost the same, regardless of Ta thickness. In addition, the similar Co atom distributions means that etching level 5 for both samples represents the CoFeB/MgO interface region, as presented
Fig. 1. Magnetic hysteresis loops (black and red lines represent in-plane direction and out-of-plane direction, respectively) for various Ta thickness samples at different annealing temperatures (a) 300 °C, (b) 325 °C, (c) 350 °C and (d) 375 °C. Numbers in each graph indicate Ta layer thickness used in this experiment.
S. Yang et al. / Thin Solid Films 587 (2015) 39–42
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Fig. 3. (a) Ta (tTa)/CoFeB/MgO/W stack schematic illustration for XPS analysis, where tTa is 1.5 nm and 4.5 nm. (b) Depth profiling analyses for Ta and Co atoms in Samples A and B. Expected locations in real configurations along each etching time are shown behind the graph.
Fig. 2. (a) Magnetic moment per unit volume and (b) effective anisotropy energy density as a function of Ta layer thickness for Ta/CoFeB/MgO/W stacks at different annealing temperatures.
in Fig. 3b. However, a Ta 4f depth profile measurement revealed a clear difference in Ta atom distributions after thermal treatment. In particular, at the CoFeB/MgO interface region (12 s etching), a larger Ta atom amount in Sample B appeared, compared to those in Sample A, possibly confirming changes in effective anisotropy for both samples after thermal treatment (Fig. 2b). Therefore, distinctness in Ta depth profiles was possible indirect evidence for Ta thickness-dependent diffusion phenomena during annealing. A closer investigation for ultrathin Ta thickness samples (Ta 0.75 nm stacks) exhibited an easy transition from PMA to in-plane magnetic anisotropy features at the relatively lower annealing temperature of 325 °C, as shown in Fig. 4. This is different from the tendency shown in Fig. 2. The difference may be related to the fact that ultra thin Ta thickness (0.75 nm) provided an unclear Ta interface. It possibly led to a ragged CoFeB interface, inducing a lower interfacial anisotropy (Ki) value between CoFeB and MgO layers at a relatively lower annealing temperature. In addition, Ta atom diffusion influence in an ultra thin 0.75 nm sample is less than those in other thicker samples. However, since this sample's initial interface anisotropy feature was not better than thick samples, the 0.75 nm Ta thick sample did not allow for better PMA properties, even at the relatively lower 325 °C annealing temperature, compared to thick samples.
To confirm our observations for various Ta thicknesses, the deadlayer in the CoFeB layer was obtained as a function of CoFeB thickness for 0.75 nm, 1.5 nm, and 4.5 nm Ta thick samples, which were not thermally treated. As shown in Fig. 5, dotted lines represent first order linear fitting for each sample, and vertical solid lines, including corresponding numbers, indicate the representative dead-layer thicknesses at the linear fitting x axis-intercept. The dead layer thickness of the Ta 0.75 nm sample was 0.39 nm, whereas 1.5 nm and 4.5 nm samples revealed relatively thin 0.26 nm dead-layers. The thick dead-layer presence for the 0.75 nm sample could result from a large surface roughness, which might cause relatively low interface anisotropy value (Ki) due to rough interface between CoFeB and MgO layers.
4. Conclusion In conclusion, we report Ta thickness influence on Ta/CoFeB/MgO stack magnetic features at various annealing temperatures. Magnetic hysteresis loop observations and XPS atomic depth analysis suggested that the difference in Ta atom diffusion process depended on Ta layer thickness. In addition, ultra thin Ta layers were not useful to provide stable PMA characteristics after high temperature annealing, since thin seed layers easily induced a large surface roughness, such as islands. Therefore, a proper Ta thickness or other seed layer is required for high thermal stability realization.
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S. Yang et al. / Thin Solid Films 587 (2015) 39–42
Fig. 4. Typical M–H Ta (0.75)/CoFeB (1.2)/MgO (2)/W (3) stack curve at various annealing temperatures (a) 300 °C, (b) 325 °C, (c) 350 °C, and (d) 375 °C.
Fig. 5. Magnetization saturation as a function of the CoFeB thickness for Ta (tTa)/CoFeB (x)/MgO (2)/W (3) frame, where tTa varied: 0.75 nm, 1.5 nm and 4.5 nm. Linear fit to the data is shown by dotted lines. Numbers with vertical lines indicate dead-layer thickness for different stacks.
Acknowledgments This work was partially supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2060350), and partially by the Global Ph.D. Fellowship Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2012002282). References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molna, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, X. Jiang, L. Gao, J.Z. Sun, S.P. Parkin, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488.
[2] Takayuki Kawahara, Riichiro Takemura, Katsuya Miura, Jun Hayakawa, Shoji Ikeda, Young Min Lee, Ryutaro Sasaki, Yasushi Goto, Kenchi Ito, Toshiyasu Meguro, Fumihiro Matsukura, Hiromasa Takahashi, Hideyuki Matsuoka, Hideo Ohno, 2 Mb SPRAM (Spin-Transfer Torque RAM) with bit-by-bit bi-directional current write and parallelizing-direction current read, IEEE J. Solid State Circ. 45 (2010) 869. [3] S. Mangin, D. Ravelosona, J.A. Katine, M.J. Carey, B.D. Terris, Eric E. Fullerton, Currentinduced magnetization reversal in nanopillars with perpendicular anisotropy, Nat. Mater. 5 (2006) 210. [4] K. Mibu, T. Shinjo, Magnetic anisotropy in Fe/rare-earth multilayers, Hyperfine Interact. 113 (1998) 287. [5] P.F. Carcia, Perpendicular magnetic anisotropy in Pd/Co and Pt/Co thin‐film layered structures, J. Appl. Phys. 63 (1988) 5066. [6] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H.D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, H. Ohno, A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction, Nat. Mater. 9 (2010) 721. [7] Shinji Yuasa, Taro Nagahama, Akio Fukushima, Yoshishige Suzuki, Koji Ando, Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions, Nat. Mater. 3 (2004) 868. [8] H.X. Yang, M. Chshiev, B. Dieny, J.H. Lee, A. Manchon, K.H. Shin, First-principles investigation of the very large perpendicular magnetic anisotropy at Fe/MgO and Co/MgO interfaces, Phys. Rev. B 84 (2011) 054401. [9] B. Rodmacq, S. Auffret, B. Dieny, S. Monso, P. Boyer, Crossovers from in-plane to perpendicular anisotropy in magnetic tunnel junctions as a function of the barrier degree of oxidation, J. Appl. Phys. 93 (2003) 7513. [10] Y. Fukumoto, H. Numata, K. Suemitsu, K. Nagahara, N. Ohshima, M. Amano, Y. Asao, H. Hada, H. Yodai, S. Tahara, Switching-field stabilization against effects of hightemperature annealing in magnetic tunnel junctions using thermally reliable NixFe100-x/Al-Oxide/Ta free layer, Jpn. J. Appl. Phys. 45 (2006) 3829. [11] N. Miyakawa, D.C. Worledge, K. Kita, Impact of Ta diffusion on the perpendicular magnetic anisotropy of Ta/CoFeB/MgO, IEEE Magn. Lett. 4 (2013) 1000104. [12] Soo Young Jang, Chun-Yeol You, S.H. Lim, S.R. Lee, Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure, J. Appl. Phys. 109 (2011) 013901. [13] Jorg Karger, Douglas M. Ruthven, Doros N. Theodorou, Diffusion in Nanoporous Materials, vol. 1Wiley-Vch, Weinheim, 2012. 25 (100–101). [14] G.K. Wertheim, P.M.Th.M. Van Attekum, S. Basu, Electronic structure of lithium graphite, Solid State Commun. 33 (1980) 1127. [15] A.B. Mandale, S. Badrinarayanan, S.K. Date, A.P.B. Sinha, Photoelectron-spectroscopic study of nickel, manganese and cobalt selenides, J. Electron Spectrosc. Relat. Phenom. 33 (1984) 61. [16] L.A. DeLouise, Investigation of the chemical mechanisms of Ta(110)/Ta(110)suboxide etch selectivity using C12 molecular beams, Surf. Sci. 324 (1995) 233.